A geochemical investigation of the origin of scheelite mineralisation, Glenorchy, New Zealand.


Paterson, Colin J.


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The Glenorchy lode field, comprising scheelite (CaWo4) bearing quartz lodes, is one of many gold tungsten and antimony mineralized lode fields within the Haast Schist of the South Island of New Zealand. The lode fields in the Otago schists are more abundant and were more productive than those elsewhere in the Haast Schist. The Otago schist belt is composed mainly of metasediments (psanunitic and pelitic schists) with subordinate amounts of metavolcanic rock and metachert; metamorphic grade ranges from pumpellyite-actinolite facies to the biotite zone of the greenschist facies, while amphibolite facies rocks occur in the Alpine schists adjacent to the Alpine Fault to the northwest.
The origin of the epigenetic Au and W mineralization has been debated since the early 1900's when the model invoking passage through the schist of metalliferous emanations from an underlying magma was in vogue. However, the absence in the schists of suitable igneous rocks, that could be interpreted as being spatially and temporally associated with the mineralization, prompted other ore genesis models. Secretion of the Au and W from the country rocks by meteoric waters was proposed, but since the 1960's, :geological opinion has favoured an origin connected with regional metamorphism.
Both geochemical and petrological approaches are utilized in this thesis in an attempt to elucidate the controls on W mineralization at Glenorchy.
In the Glenorchy field, the lodes consist of quartz-calcitescheelite veins in crushed, folded, sulphide-bearing schist bounded by clay selvages. Scheelite is sporadically distributed in the quartz veins. Ore shoots are parallel to the dip and strike of the Glenorchy Lode, the most productive lode in the field. The lodes dip 15-30° NE, cutting the SW-dipping schistosity at an angle of 60-70°, and with the subsidiary lodes comprise a subhorizontal flattened network. It is tentatively concluded that the lodes originated by passage of ore-forming fluids along pre-existing normal faults developed during the early stages of uplift of the schist pile. Both syn- and post-mineralization deformation is recognized within the lode.
The petrography and mineralogy of the unaltered Mt Judah schists are described, and this section forms the basis for later characterization of the hydrothermal alteration. Psarnmitic and thin-bedded (alternating pelitic-psarnmitic) schists are dominant on Mt Judah, and are within pumpellyite-actinolite facies. East of Bonnie Jean Creek the schists are in greenschist facies, and the abundance of pelitic schists and greenschists increases eastwards. From the composition of paragonitic muscovite, and existing experimental data on the upper stability limit of pumpellyite, the temperature of metamorphism in the Mt Judah schists 0 was about 400 c, although oxygen isotope data suggest the temperature could have been as high as 445°C,
The petrography and mineralogy of the altered schists are described, and the changes in minimal abundance and mineral composition with hydrothermal alteration are discussed. The Mineralogical alteration zone is generally 1.5-2 m \dde and is characterized by alteration of ea-silicates (amphiboles, pumpellyite, epidote) to aggregates of chlorite and calcite, serpitization of albite, and deposition of sulphides, kaolinite, and calcite. Metamorphic sulphides, stilpnonelane, sphene and rutile were als0 altered DY the hydrothermal fluids. The mineral compositions of schist minerals were not affected, except. for chlorite which displays an increased compositional range in the altered schists, with a tendency for increased MgO and decreased FeO contents. Two alteration zones arc recognized - sericite-carbonate zone and a marginal zone.
Major element and trace element analyses of the bulk schists in four traverses perpendicular to the lode show that the altered schists are depleted in CiiO, Fe 2 o3 , sio2 , Sr, and Y, and enriched in co2 , K2o, H2o, FeO, S, Rb, Cs, W, As 1 and Sb relative to the unaltered schists. Mass balance calculations suggest the ea and Sr in the scheelite, the Fe in the sulphides, and half the Si in the quartz within the lodes originated from the altered schists adjacent to the lode. S, C, As, W, Sb, Rb, Cs, and some Si were introduced to the lodes in the Mt Judah area by the ore-forming fluid from a source deeper in the schist pile.
Oxygen isotope analyses of schist and lode minerals are presented. Adjacent to the lode there is an aureole up to 10 m wide in which the values of the quartz, albite, and muscovite are high relative to those in the unaltered schists. Oxygen isotope fractionations indicate a temperature range o.f 3500 -2800 C for h 018 . hn t mineralization, o18 enrichment aureole suggests the ore-forming fluid was previously in equilibrium with schists at deeper levels in the pile at temperatures exceeding 420°C~ The isotope composition of the fluid (oo18 == 8.sro0) is compatible with both metamorphic and magmatic derivation. The isotope aureole and the decrease in oxidation state of the altered schists (indicated by increased 2+ . Fe /total Fel have resulted from interaction between the schists and relatively large volumes of fluid transported up\va:t'ds in major conduits in the schist pile. Because of the lack of spatial relationship between mineralization and contemporaneous igneous activity,it is concluded that the mineralization was a consequence of various processes associated with burial metamorphism.
The feasibility of such an marigin is examined. Dehydration and decarbonation reactions during metamorphism in schisto of higher grade than greenschist facies are capable of producing sufficient fluid consisting mainly of H2o, with considerable co2 component. It is suggested that the fluid was trapped in the schist pile during burial, and that an increase in permeability consequent upon microhydraulic fracturing induced by rapid uplift, allowed migration of the fluids to existing planes of weakness (faults) which were then opened up as main channel ways by further hydralic fracturing. Metal- enriched horizons as sources of Au, W, and Sb are considered not essential to ore genesis, as pervasive leaching of the metals from all lithologies in the schist belt during mineral transformation reactions associated with progressive metamorphism is all that is required for development of potential ore-forming fluids. The antipathetic distribution of Au and W mineralization is a consequence of depositional rather than source parameters. Rich scheelite deposits were formed in fractiones cutting ea-mineral enriched rocks such as psammitic schists of pumpellyiteactinolite to lower greenschist facies, as a result of reaction between the fluid and wall rocks. Hydrogen metasomatism associated with sericitization of feldspar may have resulted in gold deposition, together with minor scheelite deposition,' at lower crustal levels and higher temperatures. Consequently, the ore-forming fluid depositing scheelite at higher crustal levels would be relatively depleted in Au.
The lode fields appear to be preferentially developed near hinge - zones of macroscopic nappes, but the orientation of the lode structures is related to the NE,-SW.-directed extensional tectonic regime existing after the major phases of deformation and metamorphism.
The ore genesis model developed here is compared with the models for Au and w mineralization in other metamorphic belts throughout the world. It is concluded that in most of the other belts, the ore-forming vii metals and fluids originated within the metasediment-metavolcanic piles, but that igneous intrusion rather than regional metamorphism was the driving force for the mineralization process.
The ore genesis model may be useful in a general sense in future mineral exploration in the Haast Schist, but the patterns of chemical and mineralogical variation around the lodes are probably too restricted to have any immediate application.

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xx. 266p. Ill. Plates. Maps in text. 30cm.


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POLYGON ((168.417304580585125 -45.153316362439639,168.272038230924636 -44.792974796932569,168.658304091917216 -44.709261509929178,168.828564358117575 -45.052879697941094,168.417304580585125 -45.153316362439639))





Paterson, Colin J., “A geochemical investigation of the origin of scheelite mineralisation, Glenorchy, New Zealand.,” Otago Geology Theses, accessed April 16, 2021, http://theses.otagogeology.org.nz/items/show/102.

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